In recent years, a motor with small cogging torque has been required for various applications such as industrial servo motors and hoists for elevators. In focusing attention on such applications for vehicles, an electric power steering apparatus has become widespread for achieving an improvement in fuel consumption and an improvement in steering performance. Cogging torque of a motor for use in the electric power steering apparatus is transmitted to a driver via gears; and therefore, reduction in cogging torque of the motor is strongly desired in order to obtain a smooth steering feeling. In response, one possible method to reduce the cogging torque is to provide supplemental grooves in a core of a stator. Such a method is disclosed in Patent Document 1, Patent Document 2, and Patent Document 3.